The present invention relates to a discharge lamp lightening device, in particular, relates to a leakage transformer in that device. That leakage transformer doubles as a balast for restricting discharge current in a discharge lamp like a fluorescent lamp. The present invention relates, in particular, to such a leakage transformer which supplies to a pair of fluorescent lamps.
Conventionally, a balast for restricting current in a fluorescent lamp is a choke coil inserted between a power source and a lamp. A conventional choke coil balast has the disadvantages that it is heavy in weight, large in size, and also low in efficiency.
Another conventional balast for restricting current in a fluorescent lamp is an inverter balast implemented by electronic components. The present invention relates to such an inverter balast.
When a pair of fluorescent lamps are used, it is preferable that those two lamps are lighted by a single lightening device, which can also light a fluorescent lamp even when one of the fluorescent lamps is switched off or removed. Further, it is preferable that the inverter is not influenced even when one of the lamps is switched off or removed.
FIG. 1 shows a circuit diagram of a prior inverter type fluorescent lamp lightening device, in which the symbols T1 and T2 are output transformers of an inverter IV, F1 and F2 are fluorescent lamps, IV is an inverter circuit, and LB is called a balancer choke coil. The circuit of FIG. 1 has a pair of transformers T1 and T2, the secondary windings of which are coupled parallel with each other as shown in the figure. The output of the secondary winding is applied to the center tap of the choke coil LB, both ends of which are connected to each end of the fluorescent lamps F1 and F2. The other ends of the fluorescent lamps F1 and F2 are connected to the other end of the secondary winding.
It should be noted that the firing voltage of a fluorescent lamp depends upon each lamp, and therefore, two lamps F1 and F2 do not fire at the same time in a strict sense. Even when one of the lamps fires first, the other lamp can fire because of the presence of the center-tapped choke coil LB. It should be noted that if no center tapped choke coil LB were provided, the second lamp could not fire, since the discharge voltage across the fired lamp is considerably lower than the firing voltage. Due to the presence of the choke coil LB, the circuit of FIG. 1 has the advantages that it can fire two lamps even when the firing voltage of the first lamp is different from that of the second lamp, it can fire a lamp even when there is a trouble with another lamp, or even when another lamp is removed.
However, the circuit of FIG. 1 has the disadvantage that it needs two transformers T1 and T2 and the particular choke coil, which are heavy in weight, and increase the price of the device. Further, the choke coil works only when a lamp starts to fire, but it plays no role after a lamp is fired.
FIG. 2 shows another prior circuit, in which a single transformer T is used, and a pair of fluorescent lamps F1 and F2 are connected in series as shown in the figure. Further, the capacitor C is connected parallel to one (F1) of the lamps. Due to the presence of the capacitor C which has the capacitance of about 1000 pF and has considerably small impedance as compared with that of a lamp, the secondary voltage across the secondary winding of the transformer T is mainly applied to the lamp F2 which does not have a capacitor C, then, that lamp F2 is fired first. When the lamp F2 is fired, the discharge voltage across the lamp F2 is considerably lower than the firing voltage, and therefore, almost all the voltage across the secondary winding of the transformer T is applied to the first lamp F1, then, the lamp F1 is fired. The circuit of FIG. 2 has the advantages that only a single secondary winding is enough for supplying to two lamps, and no choke coil is necessary.
However, the circuit of FIG. 2 has the disadvantage that when one of the lamps, in particular the lamp F2, is removed or there is something wrong with the lamp F2, the other lamp F1 can not be fired, since the series circuit is broken.
In order to solve the above disadvantage, the present applicant proposed the Japanese utility model application No. 100588/82. FIG. 3 is the circuit diagram of that proposal, in which a pair of transformers T1 and T2 are provided to lighten a pair of lamps F1 and F2, and each transformer lightens a single lamp (F1 or F2). The circuit of FIG. 3 has the advantages that no choke coil nor capacitor is necessary, and even when one of the lamps is removed, the rest of the lamp can operate.
However, the circuit of FIG. 3 has the disadvantage that two transformers T1 and T2 are necessary.